- [Narrator] In this video,
we want to consider some different OSPF network types
and some characteristics of those network types.
First up is the Broadcast Network Type.
Here, we have a broadcast domain,
we have a subnet
and a broadcast would propagate throughout this subnet.
And because this is a Broadcast Network Type,
we could have multiple routers
that have interfaces that belong to this subnet.
And we talked earlier that
that would be a reason we might want
to elect a DR a designated router
and a BDR, a backup designated router,
and that was to avoid a full mesh of adjacencies,
which could get out of control scalability wise.
And the default Hello interval,
as we demonstrated in our timers video is 10 seconds.
And this again is the default for any sort
of an Ethernet interface.
If we have a serial interface
that's not configured for frame relay, then by default,
that's going to be a Point-to-Point Network Type.
It's the default on a serial interface again,
that's not Frame Relay.
And because there are only two routers on this link,
there's no danger of a scalability issue with adjacencies.
So there is no need to elect a DR or BDR.
So they are not going to be elected.
And like a Broadcast Network Type,
the default Hello interval is again 10 seconds.
Next up, we have an NBMA Network Type
or a Non-Broadcast Network Type.
This is what we have
by default on a Frame Relay serial interface,
where we've configured the encapsulation
to be Frame Relay,
Frame Relay can set up these PVCs
between different locations.
And we've got permanent virtual circuits here going
from R1 over to R2 and another one
to R3 and another one to R4.
And those PVCs,
those Permanent Virtual Circuits are identified
with numbers that are called DLCIs,
DataLink Connection Identifiers.
And we are going to be electing a DR
and BDR on this NBMA network type.
Something that's interesting though,
because we're not doing broadcasts,
that also implies we're not doing multicasts.
So we're not going to be able to dynamically by default,
discover our neighbors.
So what we have to do here is
to explicitly state the IP address
of our neighbor using a neighbor command.
And here the Hello timer is a bit bigger.
It defaults to 30 seconds.
And finally we have a Point-to-Multipoint Network Iype.
This could be used in a Frame Relay network as well.
And what this does, it's going to replicate a packet
that it sends out to its neighbors.
So if R1 wants to send to R2
and R3 and R4,
it's going to replicate that packet
and send it down to each
of those permanent virtual circuits,
because it's treating each of those PVCs
as a separate Point-to-Point link.
So just like the Point-to-Point links,
that we talked about earlier,
there's no need to elect a DR or BDR
because there's only two devices at the ends of this link,
and it lack the NBMA network type,
the default Hello interval is 30 seconds.
And now that we've defined these different network types,
I need you to understand that they do not have
to match necessarily on each end.
Now by default, obviously,
they're going to match but we can go in
and we can change the network type to something else.
So let's talk about some valid combinations
of network types.
Could I have a router with its interface configured
for Broadcast connected to another router
that has a Broadcast network type as well?
Sure, that's what we have by default with any sort
of Ethernet link that will be a
Broadcast-to-Broadcast network type.
We could also have Non-Broadcast-Non-Broadcast.
We could have Point-to-Point-Point-to-Point no problem.
We could even have Broadcast-Non-Broadcast.
If we do that,
that's going to require that we adjust the timers
to make sure they match
because those have different default Hello intervals
by default.
We could also do a Point-to-Point to Point-to-Multipoint
network type combination.
Again, because the timers are different here,
the default Hello intervals are different.
We need to adjust the timers
to make sure they match.
But those are valid combinations of OSPF network types.